1. Field of the Invention
This invention relates to an on-demand type ink jet recording apparatus which performs recording by discharging an ink as droplets from discharge openings at a tip end of a nozzle for discharging ink by the pressure generated by an electromechanical transducing element provided along the ink path of the nozzle.
2. Related Background Art
The above on-demand type ink jet recording apparatus or device, for example as shown in FIG. 13 is equipped with a nozzle tip end 502 for discharging or emitting ink at an end of an ink pressure chamber 501, and provided with a piezoelectric element 503 as the electromechanical transducing element provided in the vicinity of the nozzle tip end 502. By applying a driving voltage corresponding to The recording data on the piezoelectric element 503. The operation of steady state, expansion, steady state; or steady state, shrinkage, steady state is done, and ink is discharged as droplets by the pressurized force created in the ink pressure chamber 501. By such discharging, the flying ink droplets are attached onto the surface of a recording medium (recording paper, film, etc.) to form dots.
The driving voltage has a stand-up portion a, a constant value portion b and a stand-up portion c as shown in FIG. 18, shrinks the ink pressure chamber 501 at the stand-up portion a, and discharges ink droplets through the pressure thereby created. The constant value portion b maintains the shrunken state, the ink chamber 501 shrunk at the portion a is expanded at the portion c, and returned to the original state by expansion by the stand-up portion a at the stand-up portion c.
Next, by referring to. FIG. 17 as well as FIGS. 19 to 21, the discharging actuation of ink droplets by the driving voltage in FIG. 18 is to be described in detail.
As shown in FIG. 17, the ink pressure chamber 501 is shrunk at the stand-up portion a of the driving voltage, whereby the ink pressure in the ink pressure chamber 501 is elevated by .DELTA.P. Also, since a pressure difference occurs at the boundary faces 504 and 505 between the ink pressure chamber 501 and ink path, pressure wave motion is generated and propagated in the direction of the ink feeding port or opening 506 and the ink discharge opening (orifice) 507.
A while after shrinkage of the piezoelectric element 503, as shown in FIG. 19, the regions 508 and 509 on the sides near the ink feeding opening 506 and the ink discharge opening 507 from the piezoelectric element 503 are under pressure with values of 1/2.DELTA.P. The length of these two high pressure portions is approximately equal to the length Q of the piezoelectric element 503. Because of the mechanical properties (mass, elastic constant, etc.) of the piezoelectric element 503, the boundary between the high pressure portions 508, 509 can not be always marked clearly as shown in FIG. 19, but they are described in this way for the purpose of convenience. The ink positioned inside of the piezoelectric element 503 under continuous shrinkage is returned at this point to the pressure (e.g. atmospheric pressure) before shrinkage.
Here, when the voltage applied on the piezoelectric element 503 becomes the stand-up portion c in FIG. 18, the piezoelectric element 503 is expanded. For this reason, the pressure of the ink positioned inside of the piezoelectric element 503 is lowered or decreased to become -.DELTA.AP as shown in FIG. 20. Then, similarly as in the case at the moment when the piezoelectric element 503 is shrunk, two negative pressure portions 511 and 512 having a pressure of -1/2.DELTA.P with the length within the ink path of Q occur as pressure waves as shown in FIG. 21. In the above description, since the constant value portion b of the driving voltage is made to have a long time period, the high pressure or positive pressure regions 508 and 509 in FIG. 19 are made to have completely left the ink pressure chamber 501. However, practically the constant value portion b is short, and therefore the regions 508 and 509 and the negative portions 511 and 512 may sometimes overlap each other. However, since these have linear characteristics, they can be considered as classified into two cases.
Whereas, the portion of the positive pressure region 509 extrudes ink through the ink discharge opening 507 to convert its wave motion energy to the motion energy of ink droplets. The positive pressure 1/2.DELTA.P in the region 509 will not lose the energy completely, but is weakened considerably as compared with 1/2.DELTA.P and reflected against the wall surface of the nozzle tip end 502 and the ink discharging opening 507 to be directed toward the ink feeding orifice 506. On the other hand, the positive pressure 1/2.DELTA.P in the positive pressure region 508 and the negative pressure -1/2.DELTA.P in the negative pressure portions 511, 512 reciprocate within the ink pressure chamber 501. At this time, positive pressure 1/2.DELTA.P of the . region 508, when reflected at the ink feeding opening, is directed toward the nozzle tip end 502 direction as the negative pressure of -1/2.DELTA.P, while on the contrary, negative pressure -1/2.DELTA.P of the negative portion 511 is directed toward the nozzle tip end 502 direction as the positive pressure (this is because the ink feeding opening 506 is an open end). On the other hand, the negative pressure portion 512 is reflected similarly at the ink discharge opening 50? to be directed toward the ink feeding opening 506.
However, in such ink jet recording method of the prior art, since the diameter d2 of the ink discharging opening 507 is sufficiently smaller as compared with the diameter d1 of the ink pressure chamber 501, the discharging opening 507 functions not as the open end but as the closed end. For this reason, the negative pressure -1/2.DELTA.P of the portion 512 even after reflection is propagated as the negative pressure portion toward the ink feeding opening 506. Accordingly, the respective pressure waves of the positive pressure and the negative portions 511, 512 with negative pressures in the region 508 are reflected against the ink feeding opening 506 and the ink discharge opening 507 to move in reciprocating manner, and every time when it reaches the ink discharge opening 507, the meniscus 514 formed at the ink discharge opening as shown in FIG. 22 moves toward the direction 515 or the direction 516. The positive pressure, negative pressures 511 and 512 in the region 508 will move in reciprocating manner between the ink feeding orifice 506 and the ink discharging orifice 507 and will not stop until force is weakened.
For this reason, it takes a long time before discharging of the next ink, to worsen the frequency characteristic of the head. Also, the second droplet will be discharged when reaching the ink discharge opening 507 as the positive pressure wave, whereby the image quality is worsened. Further, when the negative pressure wave reaches the ink opening 507, air is imbibed to generate foam within the ink path, whereby ink discharging inability may be sometimes brought about.
For solving the above problems, one may consider to apply a second pulse voltage on the piezoelectric element 503. That is, the stand-up time of the second pulse voltage is made coincident with the time when the positive pressure 1/2.DELTA.P in the region 508 shown in FIG. 19 and FIG. 20 is reflected against the feeding opening 506 and passes through the innerside of the piezoelectric element 503 as the negative portion 517 as shown in FIG. 23, thereby cancelling the negative portion 517. However, although discharging of the second droplet can be suppressed, due to application of the second pulse voltage, two positive pressure wave motions and one negative pressure wave motion are created, and therefore it takes a long time before restoration of the meniscus, and also there is involved the inconvenience of the risk of incorporating foam.
As another method for driving an ink jet recording head of the on-demand type, there has been known, for example, the method in which a voltage is applied as shown in FIG. 24 on a piezoelectric element as an electromechanical transducing system (Japanese Patent Application Laid-open No. 62-25058). According to this driving method, the voltage as mentioned above is applied on the piezoelectric element 611 of an ink jet recording head constituted as shown in FIG. 25 by use of a circuit block 612. More specifically, first, the piezoelectric element 611 is expanded in the voltage step a, and expansion of the piezoelectric element 611 is maintained for a predetermined time period under a constant voltage b. During this time period, the meniscus 620 of the discharge opening 615 is returned slightly into the nozzle. After elapse of a predetermined time period, the piezoelectric element 611 is abruptly shrunk by the voltage step c, thereby discharging the ink droplets 614 through the orifice 615.
As the voltage waveform, the waveform as shown in FIG. 26 may be sometimes applied. This waveform has the waveform comprising the parts d, e, and f added in the process of returning to the state before actuation, which performs shrinkage and expansion of the piezoelectric element 611 so as to effect stabilization of the meniscus 620.
The driving method as described above was suitable for a recording head having a structure as shown in FIG. 25 equipped with a filter 621 at a rear end of a glass tube 622 having forming the ink path 613, which filter contributed to stabilization and early attenuation of the motion of the meniscus 620 after ink discharging by absorption of the pressure wave propagating through the ink within the ink path 613.
However, the above driving method could not be applied as such to an ink jet recording head, which is not provided with a filter 621 at the rear end of the ink path 613. Also, the above filter 621 is expensive, and also it is required to be mounted and welded at the rear end of the glass tube 622, for which a large number of steps have been required.